The fuels currently used in transport vehicles are predominately obtained from refining petroleum, a non-renewable resource. Environmental problems coupled with petroleum reserve depletion has stimulated development of renewable transportation fuels. One renewable fuel oil source is biodiesel, a clean-burning alternative fuel produced from renewable resources. Biodiesel contains no petroleum, but it can be blended in any concentration at any level with petroleum based diesel fuel to create a fuel blend. Because it has similar combustion properties as diesel, biodiesel can be used in existing compression-ignition (diesel) engines with little or no modification. Biodiesel is biodegradable, essentially nontoxic, and essentially free of sulfur and aromatic compounds, and thus can provide certain environmental advantages.
Biodiesel is essentially a mixture of methyl and/or ethyl esters of fatty acids, typically made by transesterification of fatty acid triglycerides (vegetable oils and/or animal fats) with an alcohol such as methanol and/or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerin, which is removed. In addition to saturated fatty acids, such as stearic acid (n-octadecanoic acid) and palmitic acid (n-hexadecanoic acid), these naturally occurring oils contain unsaturated fatty acids, such acids as oleic acid (cis-9-octadecenoic acid), linoleic acid (cis, cis-9,12-octadecadienoic acid), α-linolenic (cis-cis-cis-9,12,15-octadecatrienoic acid), and other fatty acids bearing one or more carbon-carbon double bonds. Alkyl ester fuels made from these oils also contains esters, typically methyl and/or ethyl esters, of these unsaturated acids.
Storage stability is a problem with alkyl ester fuels because the unsaturation makes them susceptible to oxidation. The fatty acid esters can be oxidized by atmospheric oxygen and/or be metabolized by microbes to produce shorter chain carboxylic acids that generate a characteristic foul odor and excess acidity. An alkyl ester fuel that has been significantly degraded by oxidation and/or microbiological colonization is generally referred to rancid. The odor of a rancid alkyl ester fuel is easily detected, and the acid number, generally expressed as the milligrams of potassium hydroxide consumed per gram of fuel, will be significantly elevated. Excess acidity in alkyl ester fuels causes a number of problems, including corrosion, poor combustion, elevated pour point, valve deposits, and accelerated decomposition. Thus, a need exists for ways of reducing or eliminating oxidative degradation of the alkyl esters of unsaturated fatty acids found in alkyl ester fuels.